An insole is an element inserted between a shoe and the foot to reduce local pressures in sensitive spots on the plantar surface of the foot so as to prevent or reduce pain and future damage to the foot and to the locomotor system. This is accomplished by fixing the foot in a certain position and orientation relative to the shoe, and by controlling the shape of the insole/foot interfacial surface.
Several types of insoles are widely marketed. For certain problems, a standard, pre-cut insole suffices which can be mass produced in various shoe sizes and fit into conventional shoes. However, in certain cases individually tailored shoe inserts which are shaped to the individual foot, are required. Such inserts are presently made by a process which is both laborious and time consuming. A plaster cast is taken of the affected foot and, using this cast, an insert of the proper size and shape is built by hand and fit into the shoe.
Two other methods disclosed in the literature, which are less time consuming than this method, are based on molding an insole inside the shoe. The first is a method of producing inserts for ordinary shop-bought or standard "deep shoes", disclosed by R. G. S. Platts, S. Knight and I. Jakins in an article entitled "Shoe inserts for small deformed feet", Prosthetics And Orthotics International, 1982, Vol. 6, pp. 108-110. This method involves molding the insert in the shoe either using the foot itself or a positive cast of the foot. The method includes preparing a shoe-shaped "polythene" bag, which is out, sealed and heat shrunk onto a last of approximately appropriate size for the shoe, and preparing an insole base for stiffening. The insole base is placed in the polythene shoe bag and both are placed in the shoe. If using the patient's foot directly, the foot is clothed in stockinette and covered with a shaped sock made of Ambla P072 having a polyurethane film which is painted with a release agent where adhesion to the foam is not required. If using a cast, the cast is covered with a thin latex sheath.
The components of a flexible self-generating polyurethane foam are mixed and quickly poured into the polythene bag in the shoe. The foot or cast is placed in the shoe and the correct attitude is maintained for a further two minutes until the foam hardens. The patient should bear weight on the foot.
Once the foam has hardened, the foot and the polythene bag are removed from the shoe and the insert is trimmed as necessary. A layer of performed polyurethane foam is added to complete the insert.
It is a disadvantage of this method that the uncontrolled pressure created within the shoe during molding causes a change in foot shape and placement relative to the shoe.
The second method of preparing an insole inside the shoe is disclosed in U.S. Pat. No. 3,895,405. This method comprises placing a flexible foam insole member into a shoe, heating the insole to a temperature sufficient to cause the foam to lose some of its resiliency, placing a foot in the shoe before the insole regains its resiliency, and taking steps with the foot in the shoe until the insole regains its resiliency.
Both of these methods suffer from the difficulty of fixing the position of the foot and its joints while in the shoe. Therefore, it is not possible to obtain the desired height and precise shape of the insole by the molding process itself, thus requiring additional steps of trimming or adding layers to the insole, which lengthen the process of insole production. Furthermore, since both of these methods are based on weight bearing during the molding process, they cannot be applied to those corrective concepts which are based on non-weight bearing while producing an insole.